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Technical Discussions
IceClampers
IceClampers
07:56 Apr-25-2001
Doubling

I know doubling is that the reading shows double what the actual wall thickness is but what causes it and how can it be identified if the approx wall thickness is unknown.


    
 
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Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
08:36 Apr-25-2001
Re: Doubling
: I know doubling is that the reading shows double what the actual wall thickness is but what causes it and how can it be identified if the approx wall thickness is unknown.

It's typically caused by using a transducer, instrument, and/or instrument setup that is inappropriate for the material and thickness range that you're trying to measure. With single element transducers, the first backwall echo from the test material is swallowed up by either the exitation pulse or by delay line interface echo ringdown. With dual element transducers, the first backwall echo can be too small to be detected and the second may be larger (due to the response pattern of duals and/or time dependent gain functions). In either case, the instrument reads the second backwall echo, causing a doubled thickness reading.

The first solution is to always be sure that you're using the proper gear and setup for the intended application. Consult with the manufacturer if you have doubts. Use reference standards appropriate for the testyou're doing to verify performance before you start. Also, instruments with waveform display will usually give you the necessary clues to allow you to detect the echo-jumping that causes doubling and rectify the problem.

If material thickness range is COMPLETELY unknown, I would strongly recommend using a waveform display instrument and learning how to interpret what it tells you. Otherwise, you would have no way of knowing for sure if the part you're trying to measure is outside the range of the equipment and setup that you're using and you could, potentially, get fooled.

--Tom Nelligan
Panametrics, Inc.



    
 
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Art Leach
Sales
GE Inspection Technologies, USA, Joined Apr 2001, 12

Art Leach

Sales
GE Inspection Technologies,
USA,
Joined Apr 2001
12
06:00 May-01-2001
Re: Doubling
: I know doubling is that the reading shows double what the actual wall thickness is but what causes it and how can it be identified if the approx wall thickness is unknown.
.
Doubling typically occurs in thickness gauging when attempting to measure very thin materials (below the instrument or probe capabilities). Most transducers (typically dual element) are designed to measure over a thickness range. For example, a probe has a thin measuring specification of 0.030” to a maximum of 2.00”. The sound beam is transmitted into the material bounces off the backwall and is received by the receiving side of the probe. Not only did the sound beam travel the thickness of the part, it also had to transverse across the part (from the transmitter to the receiver). This becomes more difficult as the test thickness becomes thinner. Dual element probes are designed to transmit and receive sound at a very slight angle to achieve thin measurement performance. Note : Thickness gauges using dual element probes have automatic V-path compensation to account for the additional time it takes the sound beam to get from the transmit side to the receive side of the probe and can add error into doubling thickness measurements.

When the material is too thin, the sound is transmitted into the material but the first echo, which bounces off the back wall, has not had enough time to traverse to the receiving element, thus it is not detected. The second trip of this echo does have enough time to traverse, thus the receiver receives it and because it took two trips instead of one, the thickness value measured is two times the material thickness.

Single element probes do have the issue of receiver recovery (echoes being swallowed up) which is why thickness probes typically have delay lines to remove this problem.



    
 
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